High n-type doping in GaAs using silicon suffers from its amphoteric character and free carrier concentrations are practically limited to 3 x 10(18) cm(-3). Se as group VI element should not be amphoteric and promises higher carrier concentrations. The highest Hall free carrier concentration in GaAs:Se layers grown by MOVPE is indeed approximately two times higher than when using silicon doping. However, the doping is not stable to subsequent growth at high temperatures. The effects of high Se doping of GaAs on the electrical and optical properties are discussed in conjunction with the observed change of the lattice constant. (C) 2010 Elsevier B.V. All rights reserved.